This invention generally relates to an improved acid type, resin-fortified starch adhesive, and to a method for bonding surfaces together using the adhesive.
One of the major commercial uses of starch adhesives is in the preparation of corrugated paperboard. As is well known, corrugated paperboard comprises a fluted medium and a liner adhesively joined to the tips of the fluted medium on one or both sides thereof. The adhesive is normally applied in the corrugator to either the fluted medium or the liner. In most commercial corrugators, the adhesive is applied to the tips of the fluted medium and a bond is formed when the adhesive coated tips are pressed against the inner surface of the liner with the assistance of heat and considerable pressure to form the corrugated paperboard.
Although the adhesive of the invention is useful as a multi-purpose adhesive, it finds particular utility as a corrugator adhesive and, for purposes of clarity and illustration only, the adhesive is described herein mainly in terms of a corrugator adhesive and the advantages it provides in the corrugating process.
The corrugating process and the use and operation of corrugators to carry out this process are well known in the art and the details thereof need not be repeated herein. U.S. Pat. Nos. 2,102,937 and 2,051,025 issued to J. V. Bauer describe and show a typical corrugator and corrugating process in which a starch adhesive is employed, said patents incorporated herein by reference. See in particular FIG. 1 of each patent and the portions of the patent specifications pertaining thereto.
In a typical corrugating process, the first step involves the formation of a single faced board, i.e., one having a liner joined to only one side of the fluted medium. This is normally carried out in a portion of the corrugator known as the "single facer". The preparation of single faced board and the operation of the single facer are described in detail in the Handbook of Paperboard and Board, by Robert R. A. Higham, Chapter 7, pp. 101-120, Business Books Ltd., London (1971), said publication incorporated herein by reference. See in particular FIG. 7.24 on page 120 thereof.
To prepare single faced board, a strip of paper is corrugated or fluted by means of heated fluted rolls. An adhesive is applied to the tips of the flutes on one side only and the tips then joined to a liner under the influence of pressure and heat to form the single faced paperboard.
If a double faced paperboard is desired, a second operation is carried out in which adhesive is applied to the exposed tips of the single faced board and the adhesive coated tips are then pressed against a second liner in the combining section of the corrugator under the influence of pressure and heat to form the double-walled product. The adhesive then cures to form a permanent, tough, strong bond which maintains the unitary structure of the product. The preparation of double faced board is shown and described in the two Bauer patents and also in the more recent U.S. Pat. No. 3,434,901 to Griffiths et al., said patent incorporated herein by reference. See in particular FIG. 1 thereof. Other types of corrugated paperboard such as double wall board can also be prepared in accordance with known procedures.
The selection of the corrugator adhesive is an important consideration in the successful operation of the corrugator. The adhesive finally selected is the result of numerous factors appreciated by those skilled in the art, including the type of bond required in the finished corrugated product. Because of their generally desirable adhesive properties and relatively low cost, starch adhesives are by far the most common corrugator adhesives in commercial use today.
The most common starch corrugator adhesives in commercial use today are on the alkaline side of the pH range. While such adhesives produce generally acceptable bonds, they normally are not very water-resistant. The water-resistance of the bond can be improved by incorporating into the adhesive a water-resistant thermosetting resin such as urea-formaldehyde or resorcinol-formaldehyde. However, the water-resistance though improved is still not usually suitable for applications in which a highly water-resistant bond is required. For example, corrugated board is widely used to package vegetables for shipment which are wet from prior treating with water. An example of this is the "Hydro-Cooling" process which has been developed for corn, celery, etc., and which requires that the vegetable container be dipped in or showered with water for an appreciable length of time. It will be apparent that if the adhesive bonds in the corrugated board are not water-resistant, the board would be unsatisfactory for this application.
It is known in the art that a highly water-resistant bond is obtainable if a water-resistant thermosetting resin such as urea-formaldehyde or melamine-formaldehyde is added to the starch adhesive, and if the resin-fortified starch adhesive is kept on the acid side of the pH range. Although such adhesives provide water-resistant bonds, they raise new problems. For example, acid type starches have a substantially higher gelation temperature than the alkaline starch adhesives. As a result, they take longer to gel and develop their adhesive properties, and therefore provide an adhesive having a poorer "green bond" than the alkaline starch adhesives. By "green bond" is meant the bond which forms immediately when two surfaces are first brought together, i.e., the bond responsible for initially holding the surfaces together. The green bond is in contrast to the more permanent-bond obtained after the starch has had time to gel.
In a corrugator, good green bond adhesion is very important since the green bond must hold the fluted medium and liner together until the adhesive sets to form the permanent bond. This normally requires a certain amount of time which cannot normally be provided in the corrugator itself without slowing down the speed of the corrugator. A good green bond is especially important in the single facer where the adhesive coated tips of the fluted medium and the liner are pressed together for only a very brief time as they pass between the nip of the single facer rolls.
It is known in the art that when acid type starch adhesives are used, the green bond is usually so poor that unless the corrugator speed is reduced to provide longer residence times in the nip of the single facer rolls, the fluted medium and liner tend to separate after they are joined in the single facer. In this case, interruption of corrugator operation results. Reducing corrugator speed is, of course, undesirable.
There are adhesives known in the art which possess excellent green bond or "quick-gripping" characteristics. One such adhesive is polyvinylacetate, a material which has been widely used as a corrugator adhesive in applications where it is desirable to increase corrugator speed. See Pulp and Paper, Second Edition, Vol. 3, James P. Casey, Interscience Publishers (1961), pp. 1932-1933. However, because of its relatively high cost compared to the more commonly used adhesives such as starch, it has generally been used only in specialty applications.
It has been proposed to add polyvinylacetate to acid type starch adhesives to improve their green bond characteristics and thereby obtain fasten corrugator speeds despite the acidic nature of the starch adhesive. The addition of the polyvinylacetate to the acidic starch adhesives raised several serious problems. First, it became very difficult to maintain a uniform consistency in the adhesive since the polyvinylacetate tended to settle out. Secondly, although the bonds were water-resistant, they were also quite brittle.
Uniform consistency is an important property of a corrugator adhesive. An adhesive whose composition was not uniform would produce non-uniform bonds which would not be acceptable. It is also very important that the adhesive bond in corrugated paperboard be flexible. If the bond is brittle, the fluted medium of the board has a tendency to separate from its liners upon being flexed or stressed. Since the board is flexed repeatedly as it passes through the corrugator, a brittle bond could not be tolerated. The bond must be sufficiently flexible to allow the corrugated paperboard to be flexed or stressed without damaging it but not excessively flexible to the point that the bond becomes too soft or weak and begins to flow or become non-heat resistant. For these reasons, polyvinylacetate modified acid type starch adhesives have not been widely adopted despite their improved green bonding characteristics.
It is a general object of this invention to provide an adhesive composition in which the problems associated with previous acid type starch adhesives are overcome.
It is a more specific object of the invention to provide a polyvinylacetate modified acid type starch corrugator adhesive having a highly uniform consistency and green bond characteristics which allow fast corrugator speeds, and which also produces bonds of acceptable flexibility characteristics.
These and other objects of the invention will be apparent to those skilled in the art upon a consideration of this entire specification.